Ceramic foam and process for producing the same

ABSTRACT

A ceramic foam consisting of two layers in and out, in which a highly pure ceramic shell covers up the surface layer of the fly ash, is provided along with a process for producing such a ceramic foam. The process for producing a ceramic foam  10  includes the steps of: coating the surface of raw fly ash material containing impurities  12  with alumina slurry; granulating the coated fly ash; and sintering the granulated fly ash in a manner that the granulated fly ash is heated in a furnace with raising temperature, then kept at a predetermined temperature for several hours and then, slowly cooled down to room temperature. The impurities are sealed inside the fly ash  11  not to transfer to the outside so that the surface of the fly ash is covered up with a ceramic shell  13  to form the ceramic foam  10  having two layers in and out.

BACKGROUND OF THE INVENTION

[0001] (1) Field of the Invention

[0002] The present invention relates to a ceramic foam, which isproduced starting from fly ash discharged from a thermal power plant andthe like or pulp sludge discharged from a paper mill as main rawmaterials thereof and is applied to a light aggregate for concrete, andprocess for producing the same.

[0003] (2) Description of the Related Art

[0004] In a thermal power plant in which coal is used for boiler fuel, alarge quantity of fly ash is discharged as the industrial waste.Recently, such a fly ash has been effectively used as a ceramicaggregate for concrete of building materials. The fly ash is furtherexpected to be put to practical use as reclaimed resources of a largequantity, which is free from care of being exhausted for the time being,as well as the pulp sludge.

[0005] For example, Japanese Patent Application Laid-Open No. H10-297977discloses a technique that powder such as aluminum oxide (alumina)powder is bound to the surface of fly ash followed by sintering so as toproduce a hollow ceramic foam.

[0006]FIGS. 3A and 3B are a cross sectional view schematicallyillustrating a ceramic foam 1 before and after sintering, respectively,which is produced by such a conventional process as disclosed in theJapanese Patent Application Laid-Open No. H10-297977. As shown in FIG.3A, ceramic powder 4 such as Al₂O₃, MgO and SiO₂ powder is coated on thesurface layer of fly ash 2 containing impurities 3 such as Fe₂O₃followed by granulation so as to form a ceramic powder layer. In asintering step after the granulation, the granulated fly ash is sinteredat 1100-1300° C., thereby a ceramic foam 1 having low density and highmechanical strength is produced.

[0007] The ceramic foam 1 thus produced according to the conventionalprocess has the following disadvantage.

[0008] That is, when the fly ash 2 is coated with the ceramic powder 4on the surface layer thereof, then granulated and then, sintered, asshown in FIG. 3B, the impurities 3 (such as Fe₂O₃) contained in the flyash 2 transfer to the ceramic powder 4 and fuses together with theceramic powder 4. As a result, a desired ceramic foam, i.e. a hollowparticle consisting of two layers in and out, in which a highly pureceramic shell covers up the surface layer of the fly ash, cannot beproduced.

SUMMARY OF THE INVENTION

[0009] It is therefore an objective of the present invention to solvethe above problem and to provide a high quality ceramic foam consistingof two layers in and out, in which a highly pure ceramic shell covers upthe surface layer of the fly ash, and a process for producing such aceramic foam.

[0010] In order to attain the above objective, the present invention isto provide a ceramic foam having two layers in and out consisting of: aninner layer of hollow fly ash part produced by a process, in which rawfly ash material containing impurities is coated with alumina slurry onthe surface thereof, then granulated and then, sintered; and an outerlayer of a ceramic shell part produced by said process covering up thesurface of the hollow fly ash part.

[0011] According to the constitution described above, a ceramic foam asa high quality hollow particle consisting of two layers in and out, inwhich a ceramic shell covers up the surface layer of the hollow fly ash,can be provided.

[0012] In order to attain the above objective, the present invention isalso to provide a process for producing a ceramic foam comprising thesteps of: coating the surface of raw fly ash material containingimpurities with alumina slurry; granulating the coated fly ash; andsintering the granulated fly ash in a manner that the granulated fly ashis heated in a furnace with raising temperature, then kept at apredetermined temperature for several hours and then, slowly cooled downto room temperature, whereby the impurities are sealed inside the flyash not to transfer to the outside so that the surface of the fly ash iscovered up with a ceramic shell to form the ceramic foam having twolayers in and out.

[0013] According to the process described above, the impuritiescontained in the fly ash are prevented from transferring to the aluminaslurry, which is coated on the outer surface of the fly ash, and sealedinside the fly ash. As a result, a ceramic foam as a hollow particleconsisting of two layers in and out, in which a ceramic shell uniformlycovers up the surface of the hollow fly ash, can be provided.

[0014] Preferably, in the step of sintering, a rate of the heating is 5°C./ minute, the predetermined temperature ranges from 1350° C. to 1460°C., the several hours is three hours, and a rate of the slow cooling is5° C./ minute.

[0015] According to the process described above, the impuritiescontained in the fly ash are prevented from transferring to the aluminaslurry and from fusing together with the alumina slurry.

[0016] Preferably, air is introduced into the furnace with the amount of100 mL/minute per amount of the granulated fly ash 2.0 g in the step ofsintering.

[0017] According to the introduction of air with the appropriate amountas described above, a process in which the alumina slurry forms theceramic shell can be effectively promoted.

[0018] Preferably, the alumina slurry has a composition consisting of65-70 wt % of aluminum oxide, 2-3 wt % of ammonium polycarboxylate as adispersing agent, 2-3 wt % of acrylic emulsion as a binder, and 25-30 wt% of ion exchanger.

[0019] By setting up the composition of the alumina slurry as describedabove, the ammonium polycarboxylate as a dispersing agent prevents thealumina slurry particles from agglomerateing with each other, theacrylic emulsion as a binder promotes the sintering of the aluminapowder, and the ion exchanger acts as a solvent of the alumina slurry,thereby the desired ceramic foam as a hollow particle in accordance withthe particle design.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a cross sectional view schematically illustrating aceramic foam produced by a process according to a preferred embodimentof the present invention;

[0021]FIG. 2 is a graph illustrating a relation between sinteringtemperature and sintering time in a preferred embodiment of the presentinvention;

[0022]FIG. 3A is a cross sectional view schematically illustrating aceramic foam before sintering produced by a conventional process; and

[0023]FIG. 3B is a cross sectional view schematically illustrating aceramic foam after sintering produced by a conventional process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] In the following, a ceramic foam and a process for producing thesame according to the preferred embodiments of the present inventionwill be explained with reference to the attached drawings.

[0025]FIG. 1 is a cross sectional view schematically illustrating aceramic foam 10 produced by a process according to a preferredembodiment of the present invention. Fly ash 11 as the main raw materialis prepared and alumina slurry in which aluminum oxide (alumina) issubjected to slurrying is also prepared. To prepare and use the aluminaslurry for producing a ceramic foam in the present invention isdifferent from that alumina powder is used in a conventional technique.The fly ash 11 is coated with thus prepared alumina slurry on thesurface thereof and then granulated. In order to uniformly coat thealumina slurry on the surface of the fly ash 11, the alumina slurry hasthe following composition.

[0026] A dispersing agent such as ammonium polycarboxylate, a bindersuch as acrylic emulsion, and ion exchanger such as ion exchange waterand organic solvent are added to alumina powder, thereby the aluminaslurry based on a particle design is prepared.

[0027] Two to three wt %, preferably 3 wt %, of ammonium polycarboxylateas a dispersing agent is added to 65-70 wt %, preferably 65 wt %, ofaluminum oxide. The addition of the ammonium polycarboxylate improvesthe dispersibility of the aluminum oxide particles in the aluminaslurry. Then, 2 to 3 wt %, preferably 2 wt %, of acrylic emulsion as abinder is added thereto. The addition of the acrylic emulsion improvethe sintering property of the aluminum oxide. Further, 25-30 wt %,preferably 30 wt % of ion exchange water is added thereto. The additionof the ion exchange water allows the dispersing agent and the binder tospread over every surface of the aluminum oxide powder, therebypromoting the formation of the alumina layer on the fly ash. The weightratio of the fly ash to the aluminum oxide is set up, for example, 2 to3.

[0028] The fly ash is coated with the alumina slurry described above,then granulated and then, dried at about 25° C. Then, the driedgranulated particles are sintered in an electric furnace. FIG. 2 is agraph illustrating a relation between the sintering temperature andsintering time. That is, the sintering condition is as follows.

[0029] Normally, the dried granulated particles are heated with a lowheating rate of 5° C./minute from room temperature (15 to 35° C.) to atemperature ranging from 1350° C.-1460° C.

[0030] When the temperature reaches, for example, 1450° C., thegranulated particles is kept heated at 1450° C. for about three hours.

[0031] Thereafter, the granulated particles are cooled down to roomtemperature with a low cooling rate of 5° C./minute.

[0032] Thus, the dried granulated particles are slowly heated, then keptheated for a predetermined period of time and then, slowly cooled down,thereby the alumina slurry is prevented from fusing together with thefly ash 11. Therefore, in contrast to the conventional process, theimpurities 12 contained in the fly ash 11 are prevented fromtransferring to the alumina slurry and from fusing together with thealumina slurry in the process according to the present invention. Thatis, a highly pure ceramic shell 13 can be formed on the surface of thefly ash 11 and a high quality ceramic foam 10 of a hollow particlehaving two layers in and out can be produced.

[0033] Preferably, air is introduced into the furnace, for example, withthe amount of 100 mL/minute per amount of the granulated fly ash 2.0 gwhile the dried granulated particles is sintered in the furnace, therebygaseous impurities are prevented from adhereing to the surface of thegranulated particle during the sintering and the formation of theceramic shell 13 can be effectively promoted.

[0034] The aforementioned preferred embodiments are described to aid inunderstanding the present invention and variations may be made by oneskilled in the art without departing from the spirit and scope of thepresent invention.

What is claimed is:
 1. A ceramic foam having two layers in and outconsisting of: an inner layer of hollow fly ash part produced by aprocess, in which raw fly ash material containing impurities is coatedwith alumina slurry on the surface thereof, then granulated and then,sintered; and an outer layer of a ceramic shell part produced by saidprocess covering up the surface of the hollow fly ash part.
 2. A processfor producing a ceramic foam comprising the steps of: coating thesurface of raw fly ash material containing impurities with aluminaslurry; granulating the coated fly ash; and sintering the granulated flyash in a manner that the granulated fly ash is heated in a furnace withraising temperature, then kept at a predetermined temperature forseveral hours and then, slowly cooled down to room temperature, wherebythe impurities are sealed inside the fly ash not to transfer to theoutside so that the surface of the fly ash is covered up with a ceramicshell to form the ceramic foam having two layers in and out.
 3. Theprocess for producing a ceramic foam according to claim 2, wherein inthe step of sintering a rate of the heating is 5° C./minute, thepredetermined temperature ranges from 1350° C. to 1460° C., the severalhours is three hours, and a rate of the slow cooling is 5° C./minute. 4.The process for producing a ceramic foam according to claim 2, whereinair is introduced into the furnace with the amount of 100 mL/ minute peramount of the granulated fly ash 2.0 g in the step of sintering.
 5. Theprocess for producing a ceramic foam according to claim 3, wherein airis introduced into the furnace with the amount of 100 mL/ minute peramount of the granulated fly ash 2.0 g in the step of sintering.
 6. Theprocess for producing a ceramic foam as claimed in any one of claims2-5, wherein the alumina slurry has a composition consisting of 65-70 wt% of aluminum oxide, 2-3 wt % of ammonium polycarboxylate as adispersing agent, 2-3 wt % of acrylic emulsion as a binder, and 25-30 wt% of ion exchanger.